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-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE html>
-<html xmlns="http://www.w3.org/1999/xhtml">
- <body>
- <h1>Launch security with AMD SEV</h1>
-
- <ul id="toc"></ul>
-
- <p>
- Storage encryption in modern public cloud computing is a common practice.
- However, from the point of view of a user of these cloud workloads, a
- significant amount of trust needs to be put in the cloud platform security as
- well as integrity (was the hypervisor tampered?). For this reason there's
ever
- rising demand for securing data in use, i.e. memory encryption.
- One of the solutions addressing this matter is AMD SEV.
- </p>
-
- <h2>AMD SEV</h2>
- <p>
- SEV (Secure Encrypted Virtualization) is a feature extension of AMD's SME
(Secure
- Memory Encryption) intended for KVM virtual machines which is supported
- primarily on AMD's EPYC CPU line. In contrast to SME, SEV uses a unique
memory encryption
- key for each VM. The whole encryption of memory pages is completely transparent
- to the hypervisor and happens inside dedicated hardware in the on-die memory
controller.
- Each controller includes a high-performance Advanced Encryption Standard
- (AES) engine that encrypts data when it is written to DRAM and decrypts it
- when read.
-
- For more details about the technology itself, you can visit
- <a href="https://developer.amd.com/sev/">AMD's developer
portal</a>.
- </p>
-
- <h2><a id="Host">Enabling SEV on the host</a></h2>
- <p>
- Before VMs can make use of the SEV feature you need to make sure your
- AMD CPU does support SEV. You can check whether SEV is among the CPU
- flags with:
- </p>
-
- <pre>
-$ cat /proc/cpuinfo | grep sev
-...
-sme ssbd sev ibpb</pre>
-
- <p>
- Next step is to enable SEV in the kernel, because it is disabled by default.
- This is done by putting the following onto the kernel command line:
- </p>
-
- <pre>
-mem_encrypt=on kvm_amd.sev=1
- </pre>
-
- <p>
- To make the changes persistent, append the above to the variable holding
- parameters of the kernel command line in
- <code>/etc/default/grub</code> to preserve SEV settings across
reboots
- </p>
-
- <pre>
-$ cat /etc/default/grub
-...
-GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
-$ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg</pre>
-
- <p>
- <code>mem_encrypt=on</code> turns on the SME memory encryption
feature on
- the host which protects against the physical attack on the hypervisor
- memory. The <code>kvm_amd.sev</code> parameter actually enables SEV
in
- the kvm module. It can be set on the command line alongside
- <code>mem_encrypt</code> like shown above, or it can be put into a
- module config under <code>/etc/modprobe.d/</code>
- </p>
-
- <pre>
-$ cat /etc/modprobe.d/sev.conf
-options kvm_amd sev=1
- </pre>
-
- <p>
- After rebooting the host, you should see SEV being enabled in the kernel:
- </p>
-
- <pre>
-$ cat /sys/module/kvm_amd/parameters/sev
-1
- </pre>
-
- <h2><a id="Virt">Checking SEV support in the virt
stack</a></h2>
- <p>
- <b>Note: All of the commands bellow need to be run with root
privileges.</b>
- </p>
-
- <p>
- First make sure you have the following packages in the specified versions:
- </p>
-
- <ul>
- <li>
- libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
- </li>
- <li>
- QEMU >= 2.12.0
- </li>
- </ul>
- <p>
- To confirm that the virtualization stack supports SEV, run the following:
- </p>
-
- <pre>
-# virsh domcapabilities
-<domainCapabilities>
-...
- <features>
- ...
- <sev supported='yes'>
- <cbitpos>47</cbitpos>
- <reducedPhysBits>1</reducedPhysBits>
- </sev>
- ...
- </features>
-</domainCapabilities></pre>
- <p>
- Note that if libvirt was already installed and libvirtd running before enabling
SEV in the kernel followed by the host reboot you need to force libvirtd
- to re-probe both the host and QEMU capabilities. First stop libvirtd:
- </p>
-
- <pre>
-# systemctl stop libvirtd.service
- </pre>
-
- <p>
- Now you need to clean the capabilities cache:
- </p>
-
- <pre>
-# rm -f /var/cache/libvirt/qemu/capabilities/*
- </pre>
-
- <p>
- If you now restart libvirtd, it will re-probe the capabilities and if
- you now run:
- </p>
-
- <pre>
-# virsh domcapabilities
- </pre>
-
- <p>
- SEV should be listed as supported. If you still see:
- </p>
-
- <pre>
-<sev supported='no'/>
- </pre>
-
- <p>
- it means one of two things:
- <ol>
- <li>
- libvirt does support SEV, but either QEMU or the host does not
- </li>
- <li>
- you have libvirt <=5.1.0 which suffered from getting a
- <code>'Permission denied'</code> on
<code>/dev/sev</code> because
- of the default permissions on the character device which prevented
- QEMU from opening it during capabilities probing - you can either
- manually tweak the permissions so that QEMU has access to it or
- preferably install libvirt 5.1.0 or higher
- </li>
- </ol>
- </p>
-
- <h2><a id="Configuration">VM
Configuration</a></h2>
- <p>
- SEV is enabled in the XML by specifying the
- <a
href="https://libvirt.org/formatdomain.html#launchSecurity">...
</a> element. However, specifying <code>launchSecurity</code> isn't
- enough to boot an SEV VM. Further configuration requirements are discussed
- below.
- </p>
-
- <h3><a id="Machine">Machine type</a></h3>
- <p>
- Even though both Q35 and legacy PC machine types (for PC see also
- "virtio") can be used with SEV, usage of the legacy PC machine type
is
- strongly discouraged, since depending on how your OVMF package was
- built (e.g. including features like SecureBoot or SMM) Q35 may even be
- required.
- </p>
-
- <h5>Q35</h5>
-<pre>
-...
-<os>
- <type arch='x86_64'
machine='pc-q35-3.0'>hvm</type>
- ...
-</os>
-...</pre>
-
- <h5>i440fx (discouraged)</h5>
- <pre>
-...
-<os>
- <type arch='x86_64'
machine='pc-i440fx-3.0'>hvm</type>
- ...
-</os>
-...
- </pre>
-
- <h3><a id="Boot">Boot loader</a></h3>
- <p>
- SEV is only going to work with OVMF (UEFI), so you'll need to point libvirt
to
- the correct OVMF binary.
- </p>
- <pre>
-...
-<os>
- <type arch='x86_64'
machine='pc-q35-3.0'>hvm</type>
- <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
-</os>
-...</pre>
-
- <h3><a id="Memory">Memory</a></h3>
- <p>
- Internally, SEV expects that the encrypted memory pages won't be swapped
out or move
- around so the VM memory needs to be pinned in physical RAM which will be
- handled by QEMU. Apart from that, certain memory regions allocated by QEMU
- itself (UEFI pflash, device ROMs, video RAM, etc.) have to be encrypted as
- well. This causes a conflict in how libvirt tries to protect the host.
- By default, libvirt enforces a memory hard limit on each VM's cgroup in
order
- to protect the host from malicious QEMU to allocate and lock all the available
- memory. This limit corresponds to the total memory allocation for the VM given
- by <code><currentMemory></code> element. However,
trying to account for the additional
- memory regions QEMU allocates when calculating the limit in an automated
manner
- is non-deterministic. One way to resolve this is to set the hard limit
manually.
-
- <p>
- Note: Figuring out the right number so that your guest boots and isn't
killed is
- challenging, but 256MiB extra memory over the total guest RAM should suffice
for
- most workloads and may serve as a good starting point.
-
- For example, a domain with 4GB memory with a 256MiB extra hard limit would
look
- like this:
- </p>
- </p>
-
- <pre>
-# virsh edit <domain>
-<domain>
- ...
- <currentMemory unit='KiB'>4194304</currentMemory>
- <memtune>
- <hard_limit unit='KiB'>4456448</hard_limit>
- </memtune>
- ...
-</domain></pre>
- <p>
- There's another, preferred method of taking care of the limits by
- using the<code><memoryBacking></code> element along
with the
- <code><locked/></code> subelement:
- </p>
-
- <pre>
-<domain>
- ...
- <memoryBacking>
- <locked/>
- </memoryBacking>
- ...
-</domain></pre>
-
- <p>
- What that does is that it tells libvirt not to force any hard limit (well,
- unlimited) upon the VM cgroup. The obvious advantage is that one doesn't
need
- to determine the hard limit for every single SEV-enabled VM. However, there is
- a significant security-related drawback to this approach. Since no hard limit
- is applied, a malicious QEMU could perform a DoS attack by locking all of the
- host's available memory. The way to avoid this issue and to protect the
host is
- to enforce a bigger hard limit on the master cgroup containing all of the VMs
- - on systemd this is <code>machine.slice</code>.
- </p>
-
- <pre>
-# systemctl set-property machine.slice MemoryHigh=<value></pre>
-
- <p>
- To put even stricter measures in place which would involve the OOM killer, use
- <pre>
-# systemctl set-property machine.slice MemoryMax=<value></pre>
- instead. Alternatively, you can create a systemd config (don't forget
- to reload systemd configuration in this case):
- <pre>
-# cat << EOF >
/etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
-MemoryMax=<value>
-EOF</pre>
- The trade-off to keep in mind with the second approach is that the VMs
- can still perform DoS on each other.
- </p>
-
- <h3><a id="Virtio">Virtio</a></h3>
- <p>
- In order to make virtio devices work, we need to enable emulated IOMMU
- on the devices so that virtual DMA can work.
- </p>
-
- <pre>
-# virsh edit <domain>
-<domain>
- ...
- <controller type='virtio-serial' index='0'>
- <driver iommu='on'/>
- </controller>
- <controller type='scsi' index='0'
model='virtio-scsi'>
- <driver iommu='on'/>
- </controller>
- ...
- <memballoon model='virtio'>
- <driver iommu='on'/>
- </memballoon>
- <rng model='virtio'>
- <backend model='random'>/dev/urandom</backend>
- <driver iommu='on'/>
- </rng>
- ...
-<domain></pre>
-
- <p>
- If you for some reason want to use the legacy PC machine type, further changes
- to the virtio
- configuration is required, because SEV will not work with Virtio <1.0. In
- libvirt, this is handled by using the virtio-non-transitional device model
- (libvirt >= 5.2.0 required).
-
- <p>
- Note: some devices like video devices don't
- support non-transitional model, which means that virtio GPU cannot be used.
- </p>
- </p>
-
- <pre>
-<domain>
- ...
- <devices>
- ...
- <memballoon model='virtio-non-transitional'>
- <driver iommu='on'/>
- </memballoon>
- </devices>
- ...
-</domain></pre>
-
- <h2><a id="Guest">Checking SEV from within the
guest</a></h2>
- <p>
- After making the necessary adjustments discussed in
- <a href="#Configuration">Configuration</a>, the VM should
now boot
- successfully with SEV enabled. You can then verify that the guest has
- SEV enabled by running:
- </p>
-
- <pre>
-# dmesg | grep -i sev
-AMD Secure Encrypted Virtualization (SEV) active</pre>
-
- <h2><a id="Limitations">Limitations</a></h2>
- <p>
- Currently, the boot disk cannot be of type virtio-blk, instead, virtio-scsi
- needs to be used if virtio is desired. This limitation is expected to be lifted
- with future releases of kernel (the kernel used at the time of writing the
- article is 5.0.14).
- If you still cannot start an SEV VM, it could be because of wrong SELinux label
on the <code>/dev/sev</code> device with selinux-policy <3.14.2.40
which prevents QEMU from touching the device. This can be resolved by upgrading the
package, tuning the selinux policy rules manually to allow svirt_t to access the device
(see <code>audit2allow</code> on how to do that) or putting SELinux into
permissive mode (discouraged).
- </p>
-
- <h2><a id="Examples">Full domain XML
examples</a></h2>
-
- <h5>Q35 machine</h5>
- <pre>
-<domain type='kvm'>
- <name>sev-dummy</name>
- <memory unit='KiB'>4194304</memory>
- <currentMemory unit='KiB'>4194304</currentMemory>
- <memoryBacking>
- <locked/>
- </memoryBacking>
- <vcpu placement='static'>4</vcpu>
- <os>
- <type arch='x86_64'
machine='pc-q35-3.0'>hvm</type>
- <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
-
<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
- </os>
- <features>
- <acpi/>
- <apic/>
- <vmport state='off'/>
- </features>
- <cpu mode='host-model' check='partial'>
- <model fallback='allow'/>
- </cpu>
- <clock offset='utc'>
- <timer name='rtc' tickpolicy='catchup'/>
- <timer name='pit' tickpolicy='delay'/>
- <timer name='hpet' present='no'/>
- </clock>
- <on_poweroff>destroy</on_poweroff>
- <on_reboot>restart</on_reboot>
- <on_crash>destroy</on_crash>
- <pm>
- <suspend-to-mem enabled='no'/>
- <suspend-to-disk enabled='no'/>
- </pm>
- <devices>
- <emulator>/usr/bin/qemu-kvm</emulator>
- <disk type='file' device='disk'>
- <driver name='qemu' type='qcow2'/>
- <source file='/var/lib/libvirt/images/sev-dummy.qcow2'/>
- <target dev='sda' bus='scsi'/>
- <boot order='1'/>
- </disk>
- <controller type='virtio-serial' index='0'>
- <driver iommu='on'/>
- </controller>
- <controller type='scsi' index='0'
model='virtio-scsi'>
- <driver iommu='on'/>
- </controller>
- <interface type='network'>
- <mac address='52:54:00:cc:56:90'/>
- <source network='default'/>
- <model type='virtio'/>
- <driver iommu='on'/>
- </interface>
- <graphics type='spice' autoport='yes'>
- <listen type='address'/>
- <gl enable='no'/>
- </graphics>
- <video>
- <model type='qxl'/>
- </video>
- <memballoon model='virtio'>
- <driver iommu='on'/>
- </memballoon>
- <rng model='virtio'>
- <driver iommu='on'/>
- </rng>
- </devices>
- <launchSecurity type='sev'>
- <cbitpos>47</cbitpos>
- <reducedPhysBits>1</reducedPhysBits>
- <policy>0x0003</policy>
- </launchSecurity>
-</domain></pre>
-
- <h5>PC-i440fx machine:</h5>
- <pre>
-<domain type='kvm'>
- <name>sev-dummy-legacy</name>
- <memory unit='KiB'>4194304</memory>
- <currentMemory unit='KiB'>4194304</currentMemory>
- <memtune>
- <hard_limit unit='KiB'>5242880</hard_limit>
- </memtune>
- <vcpu placement='static'>4</vcpu>
- <os>
- <type arch='x86_64'
machine='pc-i440fx-3.0'>hvm</type>
- <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
-
<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
- <boot dev='hd'/>
- </os>
- <features>
- <acpi/>
- <apic/>
- <vmport state='off'/>
- </features>
- <cpu mode='host-model' check='partial'>
- <model fallback='allow'/>
- </cpu>
- <clock offset='utc'>
- <timer name='rtc' tickpolicy='catchup'/>
- <timer name='pit' tickpolicy='delay'/>
- <timer name='hpet' present='no'/>
- </clock>
- <on_poweroff>destroy</on_poweroff>
- <on_reboot>restart</on_reboot>
- <on_crash>destroy</on_crash>
- <pm>
- <suspend-to-mem enabled='no'/>
- <suspend-to-disk enabled='no'/>
- </pm>
- <devices>
- <emulator>/usr/bin/qemu-kvm</emulator>
- <disk type='file' device='disk'>
- <driver name='qemu' type='qcow2'/>
- <source
file='/var/lib/libvirt/images/sev-dummy-seabios.qcow2'/>
- <target dev='sda' bus='sata'/>
- </disk>
- <interface type='network'>
- <mac address='52:54:00:d8:96:c8'/>
- <source network='default'/>
- <model type='virtio-non-transitional'/>
- </interface>
- <serial type='pty'>
- <target type='isa-serial' port='0'>
- <model name='isa-serial'/>
- </target>
- </serial>
- <console type='pty'>
- <target type='serial' port='0'/>
- </console>
- <input type='tablet' bus='usb'>
- <address type='usb' bus='0' port='1'/>
- </input>
- <input type='mouse' bus='ps2'/>
- <input type='keyboard' bus='ps2'/>
- <graphics type='spice' autoport='yes'>
- <listen type='address'/>
- <gl enable='no'/>
- </graphics>
- <video>
- <model type='qxl' ram='65536' vram='65536'
vgamem='16384' heads='1' primary='yes'/>
- </video>
- <memballoon model='virtio-non-transitional'>
- <driver iommu='on'/>
- </memballoon>
- <rng model='virtio-non-transitional'>
- <driver iommu='on'/>
- </rng>
- </devices>
- <launchSecurity type='sev'>
- <cbitpos>47</cbitpos>
- <reducedPhysBits>1</reducedPhysBits>
- <policy>0x0003</policy>
- </launchSecurity>
-</domain></pre>
- </body>
-</html>
diff --git a/docs/kbase/launch_security_sev.rst b/docs/kbase/launch_security_sev.rst
new file mode 100644
index 0000000000..4387ae64b0
--- /dev/null
+++ b/docs/kbase/launch_security_sev.rst
@@ -0,0 +1,529 @@
+============================
+Launch security with AMD SEV
+============================
+
+.. contents::
+
+Storage encryption in modern public cloud computing is a common
+practice. However, from the point of view of a user of these cloud
+workloads, a significant amount of trust needs to be put in the cloud
+platform security as well as integrity (was the hypervisor tampered?).
+For this reason there's ever rising demand for securing data in use,
+i.e. memory encryption. One of the solutions addressing this matter is
+AMD SEV.
+
+AMD SEV
+=======
+
+SEV (Secure Encrypted Virtualization) is a feature extension of AMD's
+SME (Secure Memory Encryption) intended for KVM virtual machines which
+is supported primarily on AMD's EPYC CPU line. In contrast to SME, SEV
+uses a unique memory encryption key for each VM. The whole encryption of
+memory pages is completely transparent to the hypervisor and happens
+inside dedicated hardware in the on-die memory controller. Each
+controller includes a high-performance Advanced Encryption Standard
+(AES) engine that encrypts data when it is written to DRAM and decrypts
+it when read. For more details about the technology itself, you can
+visit `AMD's developer portal <https://developer.amd.com/sev/>`__.
+
+Enabling SEV on the host
+========================
+
+Before VMs can make use of the SEV feature you need to make sure your
+AMD CPU does support SEV. You can check whether SEV is among the CPU
+flags with:
+
+::
+
+ $ cat /proc/cpuinfo | grep sev
+ ...
+ sme ssbd sev ibpb
+
+Next step is to enable SEV in the kernel, because it is disabled by
+default. This is done by putting the following onto the kernel command
+line:
+
+::
+
+ mem_encrypt=on kvm_amd.sev=1
+
+To make the changes persistent, append the above to the variable holding
+parameters of the kernel command line in ``/etc/default/grub`` to
+preserve SEV settings across reboots
+
+::
+
+ $ cat /etc/default/grub
+ ...
+ GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
+ $ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg
+
+``mem_encrypt=on`` turns on the SME memory encryption feature on the
+host which protects against the physical attack on the hypervisor
+memory. The ``kvm_amd.sev`` parameter actually enables SEV in the kvm
+module. It can be set on the command line alongside ``mem_encrypt`` like
+shown above, or it can be put into a module config under
+``/etc/modprobe.d/``
+
+::
+
+ $ cat /etc/modprobe.d/sev.conf
+ options kvm_amd sev=1
+
+After rebooting the host, you should see SEV being enabled in the
+kernel:
+
+::
+
+ $ cat /sys/module/kvm_amd/parameters/sev
+ 1
+
+
+Checking SEV support in the virt stack
+======================================
+
+**Note: All of the commands bellow need to be run with root
+privileges.**
+
+First make sure you have the following packages in the specified
+versions:
+
+- libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
+- QEMU >= 2.12.0
+
+To confirm that the virtualization stack supports SEV, run the
+following:
+
+::
+
+ # virsh domcapabilities
+ <domainCapabilities>
+ ...
+ <features>
+ ...
+ <sev supported='yes'>
+ <cbitpos>47</cbitpos>
+ <reducedPhysBits>1</reducedPhysBits>
+ </sev>
+ ...
+ </features>
+ </domainCapabilities>
+
+Note that if libvirt was already installed and libvirtd running before
+enabling SEV in the kernel followed by the host reboot you need to force
+libvirtd to re-probe both the host and QEMU capabilities. First stop
+libvirtd:
+
+::
+
+ # systemctl stop libvirtd.service
+
+Now you need to clean the capabilities cache:
+
+::
+
+ # rm -f /var/cache/libvirt/qemu/capabilities/*
+
+If you now restart libvirtd, it will re-probe the capabilities and if
+you now run:
+
+::
+
+ # virsh domcapabilities
+
+SEV should be listed as supported. If you still see:
+
+::
+
+ <sev supported='no'/>
+
+it means one of two things:
+
+#. libvirt does support SEV, but either QEMU or the host does not
+#. you have libvirt <=5.1.0 which suffered from getting a
+ ``'Permission denied'`` on ``/dev/sev`` because of the default
+ permissions on the character device which prevented QEMU from opening
+ it during capabilities probing - you can either manually tweak the
+ permissions so that QEMU has access to it or preferably install
+ libvirt 5.1.0 or higher
+
+VM Configuration
+================
+
+SEV is enabled in the XML by specifying the
+`<launchSecurity> <https://libvirt.org/formatdomain.html#launchSecurity>`__
+element. However, specifying ``launchSecurity`` isn't enough to boot an
+SEV VM. Further configuration requirements are discussed below.
+
+Machine type
+------------
+
+Even though both Q35 and legacy PC machine types (for PC see also
+"virtio") can be used with SEV, usage of the legacy PC machine type is
+strongly discouraged, since depending on how your OVMF package was built
+(e.g. including features like SecureBoot or SMM) Q35 may even be
+required.
+
+Q35
+~~~
+
+::
+
+ ...
+ <os>
+ <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
+ ...
+ </os>
+ ...
+
+i440fx (discouraged)
+~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ ...
+ <os>
+ <type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
+ ...
+ </os>
+ ...
+
+Boot loader
+-----------
+
+SEV is only going to work with OVMF (UEFI), so you'll need to point
+libvirt to the correct OVMF binary.
+
+::
+
+ ...
+ <os>
+ <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
+ <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
+ </os>
+ ...
+
+Memory
+------
+
+Internally, SEV expects that the encrypted memory pages won't be swapped
+out or move around so the VM memory needs to be pinned in physical RAM
+which will be handled by QEMU. Apart from that, certain memory regions
+allocated by QEMU itself (UEFI pflash, device ROMs, video RAM, etc.)
+have to be encrypted as well. This causes a conflict in how libvirt
+tries to protect the host. By default, libvirt enforces a memory hard
+limit on each VM's cgroup in order to protect the host from malicious
+QEMU to allocate and lock all the available memory. This limit
+corresponds to the total memory allocation for the VM given by
+``<currentMemory>`` element. However, trying to account for the
+additional memory regions QEMU allocates when calculating the limit in
+an automated manner is non-deterministic. One way to resolve this is to
+set the hard limit manually.
+
+Note: Figuring out the right number so that your guest boots and isn't
+killed is challenging, but 256MiB extra memory over the total guest RAM
+should suffice for most workloads and may serve as a good starting
+point. For example, a domain with 4GB memory with a 256MiB extra hard
+limit would look like this:
+
+::
+
+ # virsh edit <domain>
+ <domain>
+ ...
+ <currentMemory unit='KiB'>4194304</currentMemory>
+ <memtune>
+ <hard_limit unit='KiB'>4456448</hard_limit>
+ </memtune>
+ ...
+ </domain>
+
+There's another, preferred method of taking care of the limits by using
+the\ ``<memoryBacking>`` element along with the ``<locked/>``
+subelement:
+
+::
+
+ <domain>
+ ...
+ <memoryBacking>
+ <locked/>
+ </memoryBacking>
+ ...
+ </domain>
+
+What that does is that it tells libvirt not to force any hard limit
+(well, unlimited) upon the VM cgroup. The obvious advantage is that one
+doesn't need to determine the hard limit for every single SEV-enabled
+VM. However, there is a significant security-related drawback to this
+approach. Since no hard limit is applied, a malicious QEMU could perform
+a DoS attack by locking all of the host's available memory. The way to
+avoid this issue and to protect the host is to enforce a bigger hard
+limit on the master cgroup containing all of the VMs - on systemd this
+is ``machine.slice``.
+
+::
+
+ # systemctl set-property machine.slice MemoryHigh=<value>
+
+To put even stricter measures in place which would involve the OOM
+killer, use
+
+::
+
+ # systemctl set-property machine.slice MemoryMax=<value>
+
+instead. Alternatively, you can create a systemd config (don't forget to
+reload systemd configuration in this case):
+
+::
+
+ # cat << EOF > /etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
+ MemoryMax=<value>
+ EOF
+
+The trade-off to keep in mind with the second approach is that the VMs
+can still perform DoS on each other.
+
+Virtio
+------
+
+In order to make virtio devices work, we need to enable emulated IOMMU
+on the devices so that virtual DMA can work.
+
+::
+
+ # virsh edit <domain>
+ <domain>
+ ...
+ <controller type='virtio-serial' index='0'>
+ <driver iommu='on'/>
+ </controller>
+ <controller type='scsi' index='0'
model='virtio-scsi'>
+ <driver iommu='on'/>
+ </controller>
+ ...
+ <memballoon model='virtio'>
+ <driver iommu='on'/>
+ </memballoon>
+ <rng model='virtio'>
+ <backend model='random'>/dev/urandom</backend>
+ <driver iommu='on'/>
+ </rng>
+ ...
+ <domain>
+
+If you for some reason want to use the legacy PC machine type, further
+changes to the virtio configuration is required, because SEV will not
+work with Virtio <1.0. In libvirt, this is handled by using the
+virtio-non-transitional device model (libvirt >= 5.2.0 required).
+
+Note: some devices like video devices don't support non-transitional
+model, which means that virtio GPU cannot be used.
+
+::
+
+ <domain>
+ ...
+ <devices>
+ ...
+ <memballoon model='virtio-non-transitional'>
+ <driver iommu='on'/>
+ </memballoon>
+ </devices>
+ ...
+ </domain>
+
+Checking SEV from within the guest
+==================================
+
+After making the necessary adjustments discussed in
+`Configuration <#Configuration>`__, the VM should now boot successfully
+with SEV enabled. You can then verify that the guest has SEV enabled by
+running:
+
+::
+
+ # dmesg | grep -i sev
+ AMD Secure Encrypted Virtualization (SEV) active
+
+Limitations
+===========
+
+Currently, the boot disk cannot be of type virtio-blk, instead,
+virtio-scsi needs to be used if virtio is desired. This limitation is
+expected to be lifted with future releases of kernel (the kernel used at
+the time of writing the article is 5.0.14). If you still cannot start an
+SEV VM, it could be because of wrong SELinux label on the ``/dev/sev``
+device with selinux-policy <3.14.2.40 which prevents QEMU from touching
+the device. This can be resolved by upgrading the package, tuning the
+selinux policy rules manually to allow svirt_t to access the device (see
+``audit2allow`` on how to do that) or putting SELinux into permissive
+mode (discouraged).
+
+Full domain XML examples
+========================
+
+Q35 machine
+-----------
+
+::
+
+ <domain type='kvm'>
+ <name>sev-dummy</name>
+ <memory unit='KiB'>4194304</memory>
+ <currentMemory unit='KiB'>4194304</currentMemory>
+ <memoryBacking>
+ <locked/>
+ </memoryBacking>
+ <vcpu placement='static'>4</vcpu>
+ <os>
+ <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
+ <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
+ <nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
+ </os>
+ <features>
+ <acpi/>
+ <apic/>
+ <vmport state='off'/>
+ </features>
+ <cpu mode='host-model' check='partial'>
+ <model fallback='allow'/>
+ </cpu>
+ <clock offset='utc'>
+ <timer name='rtc' tickpolicy='catchup'/>
+ <timer name='pit' tickpolicy='delay'/>
+ <timer name='hpet' present='no'/>
+ </clock>
+ <on_poweroff>destroy</on_poweroff>
+ <on_reboot>restart</on_reboot>
+ <on_crash>destroy</on_crash>
+ <pm>
+ <suspend-to-mem enabled='no'/>
+ <suspend-to-disk enabled='no'/>
+ </pm>
+ <devices>
+ <emulator>/usr/bin/qemu-kvm</emulator>
+ <disk type='file' device='disk'>
+ <driver name='qemu' type='qcow2'/>
+ <source file='/var/lib/libvirt/images/sev-dummy.qcow2'/>
+ <target dev='sda' bus='scsi'/>
+ <boot order='1'/>
+ </disk>
+ <controller type='virtio-serial' index='0'>
+ <driver iommu='on'/>
+ </controller>
+ <controller type='scsi' index='0'
model='virtio-scsi'>
+ <driver iommu='on'/>
+ </controller>
+ <interface type='network'>
+ <mac address='52:54:00:cc:56:90'/>
+ <source network='default'/>
+ <model type='virtio'/>
+ <driver iommu='on'/>
+ </interface>
+ <graphics type='spice' autoport='yes'>
+ <listen type='address'/>
+ <gl enable='no'/>
+ </graphics>
+ <video>
+ <model type='qxl'/>
+ </video>
+ <memballoon model='virtio'>
+ <driver iommu='on'/>
+ </memballoon>
+ <rng model='virtio'>
+ <driver iommu='on'/>
+ </rng>
+ </devices>
+ <launchSecurity type='sev'>
+ <cbitpos>47</cbitpos>
+ <reducedPhysBits>1</reducedPhysBits>
+ <policy>0x0003</policy>
+ </launchSecurity>
+ </domain>
+
+PC-i440fx machine
+-----------------
+
+::
+
+ <domain type='kvm'>
+ <name>sev-dummy-legacy</name>
+ <memory unit='KiB'>4194304</memory>
+ <currentMemory unit='KiB'>4194304</currentMemory>
+ <memtune>
+ <hard_limit unit='KiB'>5242880</hard_limit>
+ </memtune>
+ <vcpu placement='static'>4</vcpu>
+ <os>
+ <type arch='x86_64'
machine='pc-i440fx-3.0'>hvm</type>
+ <loader readonly='yes'
type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
+ <nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
+ <boot dev='hd'/>
+ </os>
+ <features>
+ <acpi/>
+ <apic/>
+ <vmport state='off'/>
+ </features>
+ <cpu mode='host-model' check='partial'>
+ <model fallback='allow'/>
+ </cpu>
+ <clock offset='utc'>
+ <timer name='rtc' tickpolicy='catchup'/>
+ <timer name='pit' tickpolicy='delay'/>
+ <timer name='hpet' present='no'/>
+ </clock>
+ <on_poweroff>destroy</on_poweroff>
+ <on_reboot>restart</on_reboot>
+ <on_crash>destroy</on_crash>
+ <pm>
+ <suspend-to-mem enabled='no'/>
+ <suspend-to-disk enabled='no'/>
+ </pm>
+ <devices>
+ <emulator>/usr/bin/qemu-kvm</emulator>
+ <disk type='file' device='disk'>
+ <driver name='qemu' type='qcow2'/>
+ <source file='/var/lib/libvirt/images/sev-dummy-seabios.qcow2'/>
+ <target dev='sda' bus='sata'/>
+ </disk>
+ <interface type='network'>
+ <mac address='52:54:00:d8:96:c8'/>
+ <source network='default'/>
+ <model type='virtio-non-transitional'/>
+ </interface>
+ <serial type='pty'>
+ <target type='isa-serial' port='0'>
+ <model name='isa-serial'/>
+ </target>
+ </serial>
+ <console type='pty'>
+ <target type='serial' port='0'/>
+ </console>
+ <input type='tablet' bus='usb'>
+ <address type='usb' bus='0' port='1'/>
+ </input>
+ <input type='mouse' bus='ps2'/>
+ <input type='keyboard' bus='ps2'/>
+ <graphics type='spice' autoport='yes'>
+ <listen type='address'/>
+ <gl enable='no'/>
+ </graphics>
+ <video>
+ <model type='qxl' ram='65536' vram='65536'
vgamem='16384' heads='1' primary='yes'/>
+ </video>
+ <memballoon model='virtio-non-transitional'>
+ <driver iommu='on'/>
+ </memballoon>
+ <rng model='virtio-non-transitional'>
+ <driver iommu='on'/>
+ </rng>
+ </devices>
+ <launchSecurity type='sev'>
+ <cbitpos>47</cbitpos>
+ <reducedPhysBits>1</reducedPhysBits>
+ <policy>0x0003</policy>
+ </launchSecurity>
+ </domain>
--
2.23.0